In an electric power steering apparatus the steering assist force is applied to a steering mechanism by driving an electric motor in accordance with steering torque that is applied to a steering wheel by a driver. An electronic control unit with an inverter controls the motor. The inverter feds the motor with sinusoidal motor parameters (current, voltage, magnetic flux) for torque generation. Usually, the feeding of the electric motor is monitored for faults by current feedback measurements. These current measurements can fail abruptly. Conventionally, abnormalities in the current measurement are detected by comparing the signals of two current measurement channels and, if the difference is greater than a specified limit, a shutdown is triggered. In this case, the system cannot decide which current is usable for control. The steering assist force is no longer present which is uncomfortable for the driver.
EP 1 737 116 A1 discloses a control apparatus for an electric motor in which, even in case of abnormality, where current does not flow through one phase of the electric motor or an inverter on account of disconnection or the like, a current suitable for the abnormality is caused to flow through the electric motor, and the output of the motor torque can be continued. However disadvantageously, this method does not work in case of failure of current measurement of all phases; the system shuts down, causing loss of steering assist.